(a) Field of the Invention
The present invention relates to a distributed feedback semiconductor laser device, and more in particular to the distributed feedback semiconductor laser device including, on an n-type semiconductor substrate, a layer stack having a smaller threshold current, higher initial slope efficiency and lower device resistance.
(b) Description of the Related Art
A wavelength division multiplexing (WDM) optical transmission system attracts public attention because the system can significantly increase the optical communication capacity by transmitting a plurality of optical signals having different wavelengths through a single optical fiber.
As the optical source of the WDM optical transmission system, a distributed feedback semiconductor laser device (hereinafter also referred to as “DFB laser”) having an excellent operatability at a single wavelength is generally used.
In an uncooled DFB, the temperature control of the device or the cooling of the DFB laser with a device cooling component is not conducted laser for reducing the fabrication cost. The uncooled DFB laser can be directly modulated and used for the relatively shorter distance transmission such as for the urban main line and subscribers. The uncooled DFB laser does not include the Peltier cooled device that is usually mounted as the device cooling component.
The uncooled DFB laser is required to operate in the circumstance of which temperature is uncontrolled or to continuously operate in a wider temperature range such as from −40° C. to +85° C., and to decrease its device resistance as low as possible for suppressing heat generation due to the current injection.
The DFB laser includes a structure in which the real part and the imaginary part of a refractive index in a resonator (hereinafter referred to as “diffraction grating”) are periodically changed to feedback light having a specified wavelength, thereby producing wavelength selectivity.
A lasing wavelength λDFB can be established independently from a gain peak wavelength λPL of the active layer and is defined to be λDFB=2·Λ·neff, wherein Λ is a cycle of the diffraction grating and neff is an equivalent refractive index of a waveguide. The gain peak wavelength λPL of the active layer corresponds to a peak photoluminescence wavelength.
The difference between the gain peak wavelength λPL and the lasing wavelength λDFB (Δλ=λDFB−λPL) should be strictly controlled in a specified range for obtaining excellent lasing characteristics. Δλ refers to a detuning amount.
The semiconductor laser devices are divided into the two types, one of which is an n-type device and the other is a p-type device depending on the conductivity of the semiconductor substrate. The p-type device having the layer structure on the p-type semiconductor substrate is frequently used because of the controllability of the driving circuit.
However, the light emitting strength of the p-type device is weaker due to absorption of n-type cladding layer, and the measurement of the photoluminescence wavelength λPL which is necessary for the strict control of the detuning of the DFB laser is difficult.
On the other hand, the light emitting strength sufficient for the measurement of the photoluminescence wavelength ΔPL can be obtained in the n-type device. Accordingly, for the smooth evaluation of the photoluminescence wavelength λPL in the DFB laser, the layer structure having the active layer on the n-type semiconductor substrate is frequently used.
The structure of the DFB laser includes two types. One structure forms the diffraction grating over the active layer or opposite to the semiconductor substrate (hereinafter referred to as “upper diffraction grating type”), and the other forms the diffraction grating below the active layer or between the semiconductor substrate and the active layer (hereinafter referred to as “lower diffraction grating type”).
In the structure including the n-type semiconductor substrate and the upper diffraction grating type, the detuning controllability is excellent because the cycle Λ of the diffraction grating can be established after the photoluminescence wavelength λPL of the active layer is measured.
On the other hand, the structure of the lower diffraction grating type has an advantage that the growth of the buried diffraction grating and the growth of the active layer can be simultaneously carried out to decrease the growth operations by one operation compared with the upper diffraction grating type. However, the technical difficulty is accompanied because the bandgap wavelength of the active layer should be strictly controlled for the detuning control.
In the upper diffraction grating type structure formed on the n-type semiconductor substrate that is most advantageous for the detuning control, the device resistance is likely to increase because the diffraction grating is formed in a p-type cladding layer.
The reason of increasing the device resistance in the laser device is mainly the device resistance of the p-type semiconductor layer, which is explained by the low mobility of hole carrier. The formation of the layer such as the diffraction grating having the composition different from the p-type semiconductor layer increases the energy barrier for the injected carrier, thereby increasing the device resistance.
Further, in the upper diffraction grating type structure formed on the n-type semiconductor substrate, a problem arises that a differential device resistance (dV/dI) is difficult to be clamped in a higher current injection side than a threshold current. As a result, a further problem arises that the frequency characteristic of the laser device is deteriorated at the time of the modulation to generate the performance degradation such as the reduction of the cut-off frequency at the 3 dB region determining the upper limit of the modulation frequency.